Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based, or thermal, have at least one printhead from which droplets of liquid ink are directed towards a recording medium. Within the printhead, the ink is contained in a plurality of ink conduits or channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the ends of the channels.
In a thermal ink-jet printer, the power pulse is usually produced by a heater transducer or a resistor, typically associated with one of the channels. Each resistor is individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially bulges toward the channel orifice followed by collapse of the bubble. The ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of the recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959.
Printers typically print information received from an image output device such as a personal computer. Typically, this received information is in the form of a raster scan image such as a full page bitmap or in the form of an image written in a page description language. The raster scan image includes a series of scan lines consisting of bits representing pixel information in which each scan line contains information sufficient to print a single line of information across a page in a linear fashion. Printers can print bitmap information as received or can print an image written in the page description language once converted to a bitmap consisting of pixel information.
Bitmaps printed by a printer can be printed at the resolution of the received bitmap. The printer can also modify the received bitmap and print the information at-a resolution different than the one received. In either event, it is generally believed, under most circumstances, that the higher the resolution of the printed image, or the higher the perceived resolution of the printed image, the better that image will be received by one viewing the image. Consequently, most printer manufacturers strive to print higher resolution images by either producing printheads having more ink ejecting nozzles per inch or by artificially creating the appearance of higher resolution images with printing algorithms which manipulate or alter the received bitmap.
Various methods and apparatus for printing images with scanning carriage type liquid ink printers have been developed. The following references describe these and other methods and apparatus for liquid ink printing.
U.S. Pat. No. 5,270,728, to Lund et al., describes a method for multiplying the speed-resolution product of a raster scanning or imaging device such as an ink jet printer, and a resulting data structure. A 300 dots per inch (dpi) by 600 dpi logical pixel image is mapped to a corresponding, non-overlapping physical dot image. The printer's ink jets are fired responsive to the dot image to direct individual generally spherical ink droplets onto paper at 600 dpi resolution grid timing in order to effectively double the horizontal resolution of the printed pixel image.
European Patent Application Publication No. 623 473 to Holstun et al, describes increased print resolution in the carriage scan axis of an ink-jet printer. The increased print resolution is achieved by moving the carriage of an ink-jet cartridge in the carriage scan direction to provide a first resolution in that direction which is twice the second resolution in a print media advance direction. Two smaller drops of ink are fired onto each square pixel in a single pass of the cartridge so as to provide, for example, a 600 dpi resolution in the carriage scan axis with a 300 dpi resolution in the media advance direction.
Japanese Laid Open publication number 59-109375, laid open Jun. 25, 1984, describes a method to enable printing with a high-dot density wherein dot matrix patterns are printed while reducing the pitch in the scanning direction of a head when forwardly moving the head, and the patterns are printed in the same line by upwardly or downwardly staggering the printhead by one-half dot pitch when backwardly moving the head in a wire dot serial printer.
These conventional printing methods involve either printing overlapping drops at an overlap range that can be described as chain-overlapping, from their appearance, and usually because the degree of overlap is usually less than one third the diameter of the overlapped drop. Additionally, succeeding drops are printed at a frequency that does not take into account the spreading and drying time of the preceding drop, thus limiting drop spreading physics to the characteristics of one drop at a time. As a consequence, the number of gray scale levels that can be achieved with conventional printing is limited; and poor or limited lateral spreading is more likely to cause printed images to exhibit nozzle and printhead signatures, thus resulting in less than optimum image quality.
There is therefore a need for a printing method that overcomes these shortfalls and disadvantages of conventional multiple overlapping drop printing.